Algae need not be the fittest to survive.

Two biologists working at a computer have upended some commonsense ideas of ecology. Competition for scarce resources does not necessarily winnow out redundant species, they find, and diversity need not hinge on specialization.

"It's a pretty spectacularly interesting result," comments ecologist Stephen P. Hubbell of the University of Georgia in Athens.

The researchers undertook their study to investigate the so-called paradox of the plankton. Twenty to 40 different species of phytoplankton--such as diatoms and algae--can coexist within a single cubic centimeter of water, all vying just for light and a handful of nutrients.

This observation has suggested that ecologists' justification of a species' existence in terms of an available niche could be misguided. It has even cast doubt on the principle that only the fittest survive.

Theoretical ecologist Jef Huisman of the University of Amsterdam and mathematical biologist Franz J. Weissing of the University of Groningen, both in the Netherlands, reached a new perspective on this conundrum by modeling plankton competition on a computer. They describe their work in the Nov. 25 NATURE.

Their simulations used simple equations interrelating the population size of different species and the abundance of various resources in a homogeneous environment. The species in the model differed in the effectiveness with which they scavenged various resources, so that they could excel at the exploitation of some nutrients while competing poorly for others.

Previously, modelers had simulated the competition of species for two limiting resources and found that the number of species could not exceed the number of scarce resources--two. Only a single species survived if each species was not a specialist at exploiting one of the two resources.

Huisman and Weissing investigated what happened when more than two resources were in short supply. Right at the count of three, they observed a surprising result: Species competing for three or more resources never reach a stable equilibrium. This instability dispels the strict limit of n species for n resources, the researchers report.

With three scarce nutrients, they found that species populations oscillated, and that even nine species could prosper cheek by jowl. With four or five nutrients in short supply, the fluctuations were chaotic, and still more species could coexist.

These findings didn't apply to species that are extremely specialized. Mediocre competitors that don't focus narrowly on any one nutrient are the ones that permit a highly diverse ecosystem, Huisman and Weissing observed.

Hubbell says this finding contradicts ecologists' intuitions that being different is crucial to a species' success.

In modeling primary producers such as phytoplankton and plants, which turn inorganic resources into biomass, other theorists have pointed to oscillations as a way to break the diversity barrier. To generate such oscillations, however, they had appealed to an external driving force, such as temperature variations.

"No one would have guessed that if you put three limiting resources together with many species [in a spatially featureless model environment] that they would be able to coexist" without influences from outside, says ecologist G. David Tilman of the University of Minnesota in St. Paul. "This is a major advance."

The fluctuations arise, explains Huisman, because when a species that excels at garnering one resource gains an upper hand, its burgeoning population drives down availability of another nutrient that it needs. A second species better at scavenging that nutrient then comes into its own and supplants the first.

As the balance of nutrients shifts, different sets of skills can each momentarily be advantageous. Every species gets a recurring chance to be the fittest.

Tilman characterizes Huisman and Weissing's finding as just a proof of principle, however. Many eminently reasonable justifications for diversity have emerged in recent years, he says. Only intensive field studies will reveal the extent to which each one figures in particular ecosystems.

In the meantime, the finding is a bit of nourishment for thoughtful ecologists. "I know I will think about whether this applies in the grasslands in which I work," says Tilman.